1. Field of the Invention
This invention relates to a ground drilling machine. More particularly, the present invention relates to a ground drilling machine to be used for drilling holes under the ground for the purpose of laying gas pipes, electric power cables, sheaths pipes for signal cables, optical fiber cables, water supply pipes, drainage pipes and so on without openly excavating the ground and also for drilling holes in rock beds in order to set dynamite there for blasting.
2. Description of Related Art
In recent years, efforts have been made to develop ground drilling machines for drilling holes under the ground for the purpose of laying various pipes and cables without openly excavating the ground.
FIGS. 13 and 14 of the accompanying drawings schematically illustrate such a ground drilling machine 1. The illustrated ground drilling machine 1 comprises a horizontal drill unit 2 and a working liquid supply vehicle 3 for supplying working liquid to the horizontal drill unit 2. Rods 10 are sequentially fed out from the horizontal drill unit 2 in such a way that each succeeding rod is linked to the immediately preceding rod. A pilot head 20, which is a leading body, is rotatably fitted to the front end of the leading rod 10. Thus, a drill is formed by the rods 10 and the pilot head 20 and driven by the horizontal drill unit 2.
The ground drilling machine 1 is used with a horizontal drilling technique in order to lay pipes under the ground.
Firstly, the pilot head 20 is driven into the ground from an entrance pit A at a predetermined angle of inclination (10 to 20°). Then, after correcting the direction in which the pilot head 20 is propelled to a horizontal direction, the pilot head 20 is driven to rotate and dig the ground until it gets to starting pit B. Thereafter, the pilot head 20 is driven to rotate and dig a hole from the starting pit B to a destination pit C so as to produce a leading hole D. Between the starting pit B and the destination pit C, the position, the depth from the surface, the inclination, the rotary angle and so on of the pilot head 20 are detected by means of a magnetism detector located on the ground that detects the magnetic field produced by a transmitter (sonde) contained in the pilot head 20 so as to appropriately correct the direction in which the pilot head 20 is propelled while it is digging the leading hole D. During the digging operation, working liquid that may be clean water, muddy water or bentonite solution is supplied from a working liquid supply vehicle 3 and through the rods 10 and injected from the pilot head 20.
After digging the leading hole D, the pilot head 20 fitted to the front end is replaced by a reamer 30 for broadening the leading hole and the object of underground placement E that may be a pipe or a cable to be laid is connected to the reamer 30 by way of a swivel joint. Then, the reamer 30 is rotated back, while causing the reamer 30 to eject working liquid, and the object of underground placement E is drawn into the hole, while broadening the leading hole D by means of the reamer 30.
For the operation of laying the object of underground placement E under the ground, working liquid is constantly supplied from the working liquid supply vehicle 3 and injected from the pilot head 20 or the reamer 30 in order to improve the efficiency of digging and broadening the leading hole D, that of delivering dug soil to the pits B and C and that of cooling the pilot head 20 and the reamer 30. Therefore, the rate of consumption of working liquid during the operation is remarkably high and hence the operation is costly.
The dug soil that is delivered to the pits B and C is very muddy because working liquid has been added thereto. Therefore, it is not possible to carry it out by means of ordinary dump trucks and hence it is a customary practice to draw it into dedicated container by means of a vacuuming device and carry out the containers that are filled with mud. Hence, the mud carrying out operation is costly if compared with ordinary operations of simply carrying out soil. This problem can be serious particularly when the working liquid consumption rate is high and the amount of mud to be vacuumed is large.
On the other hand, the object of underground placement E is laid between the starting pit B and the destination pit C but the operation of laying the object E inevitably entails an operation of digging a hole from the entrance pit A to the starting pit B by means of the pilot head 20. To improve the efficiency of digging a hole from the entrance pit A to the starting pit B, the distance L of the hole should be reduced as much as possible so as to complete the operation of digging a hole in which the object of underground placement E is not laid within a very short period of time.
The necessity for reducing the digging time and improving the efficiency of the hole digging operation is not limited to the operation of digging a hole from the entrance pit A to the starting pit B but is also applicable to the operation of digging a hole from the starting pit B to the destination pit C. Thus, there is a strong demand for techniques that can solve the above identified problems.